They also have a new T2 chip, just like the new Air, and every incarnation of the T2 seems to include the SSD controller for the disk that provides inline encryption, tying the chipset/storage more deeply than before. I'd say it's the same here. (This maybe isn't a strict technical limitation for not making the disk replaceable, of course, but I'm just speculating it's part of the reasoning.)

I'd guess that, unlike the SO-DIMM change here, Apple will continue to vertically integrate their storage solutions across all products as they roll out things like this. So soldered NVMe is all you're going to see from this point on, I'd imagine.

Mac Mini is hardly a portable though. In the past it had been because of thinness, but that obviously doesn't apply here.